1. Field of the Invention
The invention relates to a method for forming a carrier frequency output signal from n partial signals of a basic frequency band, wherein each of the n partial signals is attributed as an input signal to a corresponding main branch. In each corresponding main branch the partial signal is limited in amplitude, filtered and converted into an intermediate frequency partial signal, and as output signals of the main branches, the n intermediate frequency partial signals are assembled into a common sum signal. The common sum signal is limited in amplitude, subjected to a digital-analog conversion and converted into the carrier frequency output signal.
2. Description of the Related Art
In the case of radio communication systems developed according to the EDGE standard or the UMTS standard, in what is known as the single-carrier approach, complex baseband partial signals such as those for generating carrier frequency partial signals are linearly modulated and superimposed. At the same time the carrier frequency partial signals do not exhibit any fixed instantaneous power.
Due to the linear modulation, the peak values for amplitude or power which occur in each carrier frequency partial signal are significantly higher than the average power value of the carrier frequency partial signal. This yields what is known as a peak-to-average value PAR>1.
The carrier frequency partial signal is fed to a RF output amplifier with a characteristic which must have sufficient in reserve for linear amplification to avoid non-linear distortions in the amplifier output signal.
Due to the reserve requirement, such an amplifier is very complex in design and has raised power consumption. Furthermore there are consequent reductions in the overall efficiency of the transmission system.
In what is known as the multi-carrier approach, a plurality of baseband partial signals modulated using differentiated carrier frequencies is converted to an intermediate frequency range. In the range the intermediate frequency partial signals are coherently assembled into a sum signal. The sum signal is converted into a carrier frequency output signal and reaches the RF output amplifier. Here too, the peak values for amplitude or power which occur in the carrier frequency output signal are significantly higher than the average power value of the carrier frequency output signal. In this case too, the RF output amplifier must have sufficient in reserve.
In both the single-carrier and the multi-carrier approach, procedures known as clipping methods are used on the partial signals for limiting amplitude in the baseband in the former case and in the intermediate frequency range in the latter case.
During the clipping method used in the baseband, the partial signal becomes highly distorted in its frequency band. Using root raised cosine filters (also known as RRC filters), the spectrum of the distortion caused by clipping and of the partial signal itself is limited in order to reduce interference in adjacent frequency bands.
Moreover, the RRC filters form linear combinations out of past and present status values of the partial signal, since the RRC filters are used to interpolate the partial signal. This again causes a significant increase in the peak amplitude value of the partial signal that has been limited in amplitude by the clipping method.
In the case of clipping methods in the intermediate frequency range, the peak amplitude values are not raised, but serious interference is generated in adjacent frequency bands.
Two clipping methods can be used as alternatives in both the basic frequency band and the intermediate frequency band. Rectangular clipping is easily installed and limits the amplitude of in-phase and quadrature components of the partial signal independently of one another. In circular clipping an amount derived from the complex partial signal does not exceed a predefined maximum value.